The present invention relates to automotive refinish compositions and to methods for preparing and using such compositions.
Automotive topcoat finishes today are predominantly basecoat/clearcoat topcoats, in which the topcoat is applied in two layers, a first layer of a pigmented basecoat composition and a second layer of a clearcoat composition. Basecoat/clearcoat coatings are desirable for their high level of gloss and depth of color. In addition, basecoats having special effect pigments, e.g., flake pigments such as metallic and pearlescent pigment, can achieve excellent gonioapparent effect in basecoat coatings.
In order to provide optimum match to the appearance of the original finish, automotive refinish topcoats are also being applied in separate layers of basecoat and clearcoat. Unlike the original finish coating compositions, which are typically cured at temperatures of 110xc2x0 C. or higher, automotive refinish coatings must be formulated as either thermoplastic compositions or thermosetting compositions that cure at relatively low temperatures because many components of a finished vehicle cannot withstand high temperature bakes and because equipment large enough for a baked finish on a vehicle is very expensive. Nonetheless, thermosetting compositions are generally preferred as providing more durable and scratch- and mar-resistant coatings. Thermosetting refinish compositions are usually designed to cure at ambient temperatures. Although not developing full cure for hours or days, it is desirable to have the coating become xe2x80x9cdry to handlexe2x80x9d (that is, not tacky) within a reasonably short time. A coating that is dry to handle in a short time allows shorter processing times, which improves the productivity of the paint shop. Shorter dry to handle times also reduce the chance that the coating could become contaminated with airborne particulates.
It is desirable to have quick drying basecoats in the application of basecoat/clearcoat systems for an additional reason. If the applied basecoat composition layer has not dried sufficiently before the clearcoat composition is applied, then the application of the clearcoat will disturb the basecoat layer and the appearance of the basecoat will be adversely affected. In particular, the metal control of metallic basecoats will suffer due to disturbance of the flake pigment by intermixing of the coating layers at their interface.
In thermosetting automotive refinish coating compositions the curing agent reacts with the main resin or polymer at room temperatures within a reasonable amount of time without heating or with heating at low temperatures of perhaps up to 150xc2x0 F. Given the reactivity between the curing agent and the main resin or polymer at typical storage temperatures, these materials are segregated into separately stored components until just shortly before application of the coating composition to the substrate. This type of coating composition, in which the materials that react to cure the coating are segregated in two separately stored components, is referred to in the art as a xe2x80x9ctwo-componentxe2x80x9d or xe2x80x9ctwo-packagexe2x80x9d or xe2x80x9c2Kxe2x80x9d coating composition. Refinish clearcoat compositions, which are unpigmented, are often two-package compositions. Refinish basecoat compositions, on the other hand, may also be formulated as two-component compositions for each desired color or may instead be prepared from an intermix system including separately stored color components, a component having the main resin or polymer, a crosslinker component, and possibly other components.
Cost and solvent content are further concerns in formulating automotive refinish coating compositions. For example, cellulose acetate butyrate (CAB) resins have been used to shorten the dry to handle time and as rheology control additives to enhance metal control and other properties in refinish coatings, but coating compositions containing these CAB materials require an undesirably high amount of organic solvent. In addition, these CAB materials are relatively expensive and require added steps in the coatings manufacturing process. Finally, the CAB materials are specialty products that are not widely manufactured.
Certain curing agents, for example polyisocyanate crosslinkers, can be used to shorten tack-free times and for other properties, but are also relatively expensive. Using polyisocyanate crosslinkers in basecoats again increases the complexity in preparing and mixing refinish paints
It would be desirable, therefore, to have a coating composition (or intermix system for preparing such a coating composition) with a short tack-free drying time, good metal control, that is less expensive, and that could be applied with a lesser amount of regulated emissions.
The invention provides a refinish basecoat composition including at least one pigment and an hydroxyl-functional acrylic polymer. The hydroxyl-functional acrylic polymer has a number average molecular weight of at least about 6000 daltons and contains at least about 45% by weight of one or more cycloaliphatic monomers. Preferably, the refinish basecoat is free of materials reactive with the acrylic polymer. The acrylic polymer of the invention provides excellent color development, particularly in black basecoat compositions, which exhibit good jetness.
The invention further provides an intermix system for preparing the basecoat composition of the invention. The intermix system includes a plurality of color components, each containing at least one pigment dispersed by the hydroxyl-functional acrylic resin; a pigment-free component containing the hydroxyl-functional acrylic polymer; and optional further components. An automotive refinish basecoat composition of any desired color can be produced by combining the intermix system components.
Still further, the invention provides a method of refinishing a substrate, which includes steps of applying the basecoat composition of the invention to a desired area of the substrate, allowing the applied basecoat layer to dry for up to about twenty minutes, and then applying over the basecoat layer a clearcoat composition. The clearcoat composition may contain at least one component reactive with the acrylic polymer of the basecoat composition. Reaction between that reactive component of the clearcoat composition and the acrylic polymer of the basecoat promotes intercoat adhesion, even if only a minor portion of the available acrylic polymer reacts.
It is particularly desirable for the clearcoat composition to be thermosetting in order to provide a durable, scratch- and mar-resistant coating. In the composite basecoatclearcoat coating, the surface properties are provided by the clearcoat layer, thus the basecoat composition is not required to provide a hard, tough coating layer. The basecoat composition dries quickly with relatively low emissions of regulated materials and, for metallic colors, provides excellent gonioapparent effects. The composite coating of the basecoat and clearcoat has excellent adhesion to the substrate and excellent intercoat adhesion between the basecoat and clearcoat layers.